1. Field of the Invention
The present invention relates to the binuclear organometallic complexes and organic electroluminescence devices using the same, and more particularly, to binuclear organometallic complexes capable of emitting light of wavelengths from the blue range to the red range from the triplet metal-to-ligand charge transfer (MLCT) state and an organic electroluminescence device using the same as the emitting layer forming material.
2. Description of the Related Art
Generally, an organic electroluminescent device (hereinafter referred to as EL) may include an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer, and a cathode. These layers may be sequentially formed on a substrate. The hole transport layer, the light-emitting layer, and the electron transport layer may be organic layers made of organic compounds. The organic electroluminescence device having the above-described configuration may be driven as follows.
When a voltage is applied between the anode and the cathode, holes injected from the anode migrate to the light-emitting layer via the hole transport layer. Electrons emitted from the cathode are injected into the light-emitting layer via the electron transport layer. The electrons and the holes recombine in the light-emitting layer to generate excitons. As the excitons transition from an excited state to a base state, energy is transferred to luminescent molecules in the light-emitting layer, enabling the molecules to emit light and display images.
Materials for forming the light-emitting layer of the organic EL device may be classified as fluorescent materials that use a singlet or phosphorescent materials which use a triplet. The fluorescent material or the phosphorescent material or the material in combination with an appropriate host material may form a light-emitting layer. As a result of electron excitation, singlet excitons and triplet excitons are produced in the host. Statistically, the singlet excitons and the triplet excitons in an OLED are created in a ratio of about 1:3.
Conventional organic EL devices that us a fluorescent material as a material for forming a light-emitting layer may be disadvantageous in that triplets from the host may be wasted. However, organic EL devices using a phosphorescent material as a material for forming a light-emitting layer are advantageous in that singlet excitons and triplet excitons are both utilized. Such phosphorescent materials can use all of the 75% triplet excitons for emission and offers higher luminescent efficiency compared to fluorescent materials using only 25% singlet excitons for emission.
Various phosphorescent materials employing metallic complexes of Iridium or platinum as luminescent materials using triplet excitons have been discussed. However, materials satisfying requirements for realizing a full-color display of high emission efficiency or white electroluminescence with low power consumption have been restricted to ones emitting in the green and red ranges. Blue phosphorescent materials, however, have not been reported, making it difficult to achieve a full-color display, which is, in turn, becoming a barrier to the development of phospholuminescent full-color display devices.
To address the above-described problems, intensive development of blue luminescent materials is under way (See, for example, WO 02/15645 and U.S. Patent Pub. No. 2002/0064681). Also, there have been proposed organometallic complexes having a bulky functional group, e.g., a phosphine group, or a functional group having a high ligand field intensity, e.g., a cyano group, introduced thereto to increase a difference between HOMO-LUMO energy levels by transforming the molecular geometry. Specific examples of the organometallic complexes include iridium (III) complexes having cyclometalating phenyl pyridine and a phosphine group. The iridium (III) complexes are mononuclear complexes having the general formula of Ir(ppy)2P(X)3Y (X=phenyl, phenoxy or n-butyl, Y=Cl or CN.), as described in US2002/0048689 A1.